In order to study the mechanism of cell transformation, to obtain information about how to extrapolate the results obtained from the carcinogenesis experiments in animals into humans, and to compare the mutation and transformation, it is useful to have a reliable, rapid and quantitative system for simultaneous assays of malignant transformation and mutation in the cells of different species and origins including human cells. The system for malignant transformation of human diploid cells so far developed is not yet rapid and quantitative and the system to compare the induction of transformation and mutation needs to be modified. The purpose of this work is to improve the system for simultaneous assay of induction of mutation and transformation by chemical and physical carcinogens in human diploid fibroblasts and mouse cells.